1. Field of the Invention:
The present invention relates to novel catalysts for the treatment of exhaust gases from internal combustion engines and to a process for the preparation of such novel catalysts.
This invention more especially relates to novel catalysts of the type comprising a monolithic support having an active catalytic phase deposited onto the exposed surfaces thereof.
2. Description of the Prior Art:
Catalysts of the monolithic type are known to this art and include a substrate coated with a layer or film of a porous material, onto which a catalytically active phase is impregnated or deposited. These catalysts are used for the catalytic purification of the exhaust gases emanating from internal combustion engines, whether of gasoline or diesel type.
The substrate is an inert and rigid structure, designated a monolith, typically made of a ceramic or a metal.
This structure is most frequently in the form of a honeycomb containing a plurality of channels or conduits extending therethrough.
A wide variety of catalysts have been proposed to this art for the elimination of the unburned hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gases of internal combustion engines.
These catalysts are either multifunctional or oxidizing catalysts. A catalyst is multifunctional if it enables the elimination of the three principal types of pollutants contained in the exhaust gases, i.e., the unburned hydrocarbons, the nitrogen oxides and carbon monoxide. A catalyst is deemed an oxidizing catalyst if it enables oxidation of unburned hydrocarbons and the carbon monoxide contained in the exhaust gases.
These catalysts, although they have sufficient activity in certain instances to satisfy the environmental standards prevailing in such countries as the United States, Japan and in Europe, share the major disadvantage of possessing a catalytic activity which declines during their lifespan in a catalytic converter.
Multifunctional catalysts typically contain one or more precious metals, optionally in combination with one or more other metals, such as, for example, nickel, iron, zirconium, vanadium, chromium or cerium, deposited onto a monolithic support.
Thus, European Patent No. 27,069 describes a catalyst comprising a support of a refractory oxide and a catalytically active phase of cerium, iron and at least one metal selected from among iridium and rhodium, and another metal selected from among platinum and palladium.
European Patent No. 54,472 describes a multifunctional catalyst comprising an inert honeycomb substrate coated with a layer or film of a refractory oxide, with the active phase including copper, cerium or iron, at least one platinum or palladium group metal and at least one metal selected from among iridium and rhodium. The active phase is uniformly deposited or impregnated onto the exposed surface of the monolithic support, by the total immersion of the support in a solution of the precursors of the elements constituting the active phase.
The oxidizing catalysts typically contain one or more metals of the platinum type, such as platinum and palladium, and, optionally, one or more additional metals, such as nickel, iron, zirconium, vanadium, chromium or cerium.
Exemplary are the catalysts described in European Patents Nos. 100,267 and 145,584, assigned to the assignee hereof.
Also exemplary of the multifunctional or oxidizing catalysts are those described in European Patents Nos. 60,740, 126,676 and 170,588, also assigned to the assignee hereof.